01 · Polymerisation
Addition and condensation polymerisation
ADDITION
Monomers have a C=C double bond. The π bond opens and monomers add sequentially. No atoms are lost. The polymer has the same empirical formula as the monomer repeated.
n CH₂=CH₂ → -(CH₂-CH₂)ₙ-
Ethene → poly(ethene) [PE]
CONDENSATION
Monomers have two functional groups. Each bond formation expels a small molecule (H₂O or HCl). Polymers have ester, amide, or phosphodiester linkages — which are hydrolysable.
–OH + HOOC– → –O-CO– + H₂O
Ester bond formation (polyester)
The hydrolysable linkages in condensation polymers are what make polyesters (PET) susceptible to enzymatic degradation while polyolefins (PE, PP) are not. This is why plastic-eating enzymes (PETase, MHETase) work on PET but not on PE. See Reaction rates for the kinetics of enzymatic hydrolysis.
02 · Examples
Common polymers: synthetic and biological
| Polymer | Type | Monomer(s) | Uses |
|---|---|---|---|
| Polyethylene (PE) | Addition | Ethene (CH₂=CH₂) | Bottles, bags, pipes |
| Polypropylene (PP) | Addition | Propene (CH₃CH=CH₂) | Food containers, rope, carpets |
| Poly(vinyl chloride) (PVC) | Addition | Chloroethene (CH₂=CHCl) | Pipes, window frames, cable insulation |
| Nylon-6,6 | Condensation | Hexanedioic acid + 1,6-diaminohexane | Clothing, ropes, gears |
| Polyethylene terephthalate (PET) | Condensation | Terephthalic acid + ethylene glycol | Drinks bottles, polyester clothing |
| Polylactic acid (PLA) | Condensation | Lactic acid (ring-opening) | Biodegradable packaging, medical implants |
| Protein | Biological condensation | Amino acids (peptide bond) | Enzymes, structural tissue, hormones |
| DNA / RNA | Biological condensation | Nucleotides (phosphodiester bond) | Genetic information storage and expression |
Biological polymers are condensation polymers assembled by specific enzymes. Proteins are synthesised by ribosomes (see Protein synthesis); DNA is replicated by DNA polymerase (see DNA, genes and heredity). The sequence specificity of biological polymers — unique order of monomers — is what encodes genetic information and protein function.
03 · Structure and properties
How structure determines properties
The same monomer can give polymers with very different properties depending on chain length, branching, crystallinity, and cross-linking:
- HDPE vs LDPE — both polyethylene. HDPE: linear chains, high crystallinity, rigid (e.g. milk bottles). LDPE: branched chains, low crystallinity, soft and flexible (e.g. cling film).
- Natural vs vulcanised rubber — natural rubber (cis-polyisoprene) is elastic but weak. Vulcanisation forms sulfur cross-links between chains, increasing strength and temperature resistance.
- Thermoplastics vs thermosets — thermoplastics (PE, PP, PET) can be remelted and recycled because chains are not covalently bonded to each other. Thermosets (epoxy resin, Bakelite) are cross-linked and cannot be re-melted.
The intermolecular forces between polymer chains — dispersion forces in polyolefins, hydrogen bonds in nylon — determine melting point and tensile strength. See Chemical bonding for the intermolecular force hierarchy.
Polymer — common questions
- Addition polymers (PE, PP, PVC, polystyrene) form from alkenes — the π bond opens and links monomers. Condensation polymers (nylon, polyester, proteins, nucleic acids) lose a small molecule at each step. Proteins are condensation polymers of amino acids; DNA is a condensation polymer of nucleotides. See protein synthesis for biological condensation polymerisation.
- C-C backbone polymers (polyolefins) resist hydrolysis because C-C bonds are non-polar and strong. Polyesters and polyamides contain hydrolysable linkages. Engineered enzymes (PETase, FAST-PETase) can degrade PET at 72 °C in hours — see plastic-eating enzymes and microplastics in the ocean.
- Factors determining mechanical properties: chain length (higher MW = stronger), crystallinity (linear > branched chains), intermolecular forces (H-bonding in nylon > van der Waals in PE), cross-linking (vulcanised rubber is stiffer than raw rubber), and plasticisers (small molecules that space chains apart, increasing flexibility). Connects to states of matter and intermolecular forces.